Even though printed circuit boards and other substrates have gradually replaced leadframes as chip carriers of semiconductor packages, however, leadframe-based semiconductor packages still have the advantages of lower cost and high reliability. One of the leadframe-based semiconductor packages is Chip-On-Lead (COL) semiconductor package. The backside of semiconductor chip is disposed on a plurality of leads of a leadframe. Then the chip is electrically connected to the fingers of the leads by a plurality of bonding wires formed by wire bonding. Finally, an encapsulant is formed by molding to encapsulate the chip and the bonding wires where the external leads of the leads exposed and extended from the sides of the encapsulant are used as the external electrical connections. Even though COL semiconductor packages are very mature packages, however, there are still some molding defeats.
As shown in FIG. 1 and FIG. 2, a COL semiconductor package 100 comprises a plurality of first leads 110, a plurality of second leads 120, a chip 140, a plurality of first bonding wires 151, a plurality of second bonding wires 152, and an encapsulant 160. Each first lead 110 has a first external lead 111 and each second lead 120 has a second external lead 121. The first external leads 111 and the second external leads 121 are exposed and extended from two opposing sides of the encapsulant 160 and are properly bent for external electrical connections. The first leads 110 and the second leads 120 are further inwardly extended from two opposing sides of the encapsulant 160 where the lengths of the first leads 110 inside the encapsulant 160 are longer than the ones of the second leads 120 inside the encapsulant 160 for carrying the chip 140. The back surface of the chip 140 is attached to the first leads 110. The first bonding wires 151 connect from a plurality of bonding pads 141 disposed on the active surface 142 of the chip 140 to the bonding fingers of the first leads 110 and the second bonding wires 152 are bonded to the bonding fingers of the second leads 120. The encapsulant 160 encapsulates the chip 140, the bonding wires 151 and 152, parts of the first leads 110, and parts of the second leads 120.
However, as shown in FIG. 1, the chip 140 is only attached to the first leads 110 where the lengths of the first leads 110 must be long, therefore, the first leads 110 can not provide a strong structural strength to carry the chip 140. Moreover, during the formation of the encapsulant 160 by molding, the chip 140 is easily tilted or shifted due to mold flow, moreover, the first leads 110, the chip 140, or the first bonding wires 151 will easily expose from the encapsulant 160 leading to failure of the semiconductor packages.